Method of producing sulphite acid



Oct. 28, 1958 G; HAGLUND 2,858,192

METHOD OF PRODUCING SULPHITE ACID v Original Filed Aug. 5, 1947 2 Sheets-Sheet 1 Oct. 28, 1958 G. HAGLUND 2,353,192

I METHOD OF PRODUCING SULPHITE ACID Original Filed Aug. 5, 194'? 2 Sheets-Sheet 2' we Q201 U L U 2,85,192 Fatentetl 9:25;. 28, 1958 file METHQD OF PRODUCING SULPHITE ACID Gustaf I-llaglund, Storangen, Sweden Original application August 5, 1947, Serial No. 766,244,

now Patent No. 2,637,627, dated May 5, 1953. Divided and this application April 1, 1953, Serial No. 346,209

3 Claims. (Cl. 23-178) This application is a division from my application Serial No. 766,244, filed August 5, 1947, now Patent No. 2,637,627.

This invention relates to the manufacture of acid cooking liquor for use in sulfite cellulose digesters to obtain pulpand is more particularly concerned with the strengthening of raw acid liquor, produced in conventional plants, by the aid of SO -containing gases emanating periodically from such digesters.

The primary object of this invention is to provide means for substantially completely to recover the S content of relief gases blown from the digester or of blow pit gases or generally of gases emanating periodically from such digester relief operation utilise such periodically emanating at the start of the digester relief operation and the relatively low pressure gases emanating at the end of the digester relief operation and utilise such periodically emanating gases for strengthening raw acid sulfite liquor obtained by any conventional process. It is also an object to provide means for the said purpose that will be highly effective for very irregular supply of SO gas and almost constant supply of raw acid liquor but nevertheless will require apparatus having relatively small dimensions and being relatively simple in structure and cheap in price.

To obtain these and other valuable objects that will be understood from the following description, I accumulate raw acid liquor in the continuous path of such liquor to form a bulk and pass the periodically appearing SO -gases through a gas space at the top of said bulk, where I mix the liquor and gas by rapidly rotating solid bodies, preferably discs, alternatingly through the upper portion of the liquor and the gas space. The amount of raw acid liquor accumulated in said bulk together with the amount of fresh raw acid liquor supplied during said gas-emanating period of the digester being sufficient for substantially completely to absorb the S0 My invention will be better understood from the following description of a preferred embodiment illustrated in the accompanying drawings, but it is to be understood that the invention is not limited thereto.

In the drawings:

Fig. 1 illustrates diagrammatically a plant for carrying out the method. Fig. 2 shows a vertical longitudinal section of the apparatus for carrying out the method according to the invention. Fig. 3 shows the apparatus of Fig. 2 in vertical section. Fig. 4 shows the apparatus of Figs. 2 and 3 as viewed from above.

As seen from Fig. 1 the plant comprises an absorption apparatus 203 to the bottom portion of which raw acid liquor is supplied through pipe 207 from a raw acid liquor plant generally designated by numeral 201. As this may be of any conventional design it need not be described here. It should only be mentioned that such a raw acid liquor plant comprises a bisulfite producing step where limestone or similar basic material is contacted with gas from a S0 producing furnace, and as a rule also a free acid step in which the bisulfite from the first named step is contacted with said furnace gas before the latter is introduced into the bisulfite producing step so as to increase the percentage of completely free S0 of the liquor. One process for the preparation of raw sulfite acid is described in my Patent No. 2,637,627. The absorption apparatus 203 has also a gas pipe 208 through which a gas rich in S0 is supplied to the apparatus. Any residual gases escape therefrom through pipe 209. Liquor enriched in S0 is withdrawn from apparatus 203 through the overflow pipe 210 and is passed to the bottom portion of a settling tank 204 by means of a pump 211 through pipe 212. The settling tank 204 communicates at the top through overflow pipe 213 with the bottom portion of a gas reclaiming tank 205 to which relief gases from a digester, not shown, are supplied through pipe 214 to be partly absorbed in tank 205. The excess gases are passed from the top of the tank through pipe 200 to the absorption apparatus 203. The liquor still more enriched in S0 is then passed through an overflow pipe 215 to a storage tank 206 for finished cooking liquor. The con tents of this tank are pumped to the digester through pipe 218 by means of a pump 217.

The absorption apparatus 203 illustrated in Fig. l is shown more in detail in Figs. 2, 3, and 4. It comprises a tank 219 dimensioned to hold the quantity of raw acid that I wish to accumulate for absorbing the periodically emanating digester gases. Preferably, the tank 219 has a sloping bottom and a bottom discharging device 220 for sludge as well as a clean-out vent 221. The tank 219 is provided with a tightly fitting cover 222. The raw acid liquor which is produced in the raw-acid apparatus 201 is continuously passed through the pipe 223 towards the bottom of the tank. A corresponding quantity of liquor runs off continuously through the pipe 224 so that a constant liquid level 225 is formed in the tank 219 of the accumulated bulk of liquor. A hood 226 is fitted longitudinally in the cover 222 to establish on top of the bulk a gas space which is in contact with the liquor but is secluded from the surrounding atmosphere. In the longitudinal direction of the hood there passes a shaft 227, which carries a number of circular disks partly submerging below the liquid level 225 so as upon rotation of the shaft 227 by the driving means 229 to distribute liquor into the gas space. Instead of disks radially placed rods, segments of disks, blade shaped members or the like may be employed. Relief gas or the like from the digester is introduced into said gas space in the hood 226 by way of the gas conduit 230 and, owing to the. action of the rotating disks the gas is brought into a most intimate contact with the raw acid. The residue gas leaves the apparatus through the gas conduit 231, freed from its con-- tent of S0 and from a great portion of its heat. 232 designates a non-return valve, which prevents the admission of the atmospheric air when no gas is introduced through conduit 230. In one case of an ordinary plant the hood 226 may have a length of 10 feet, and the number of disks may be 29, each having an area of 21 square feet. The disks may be 600 revolutions per minute. The apparatus may with advantage be made of acid-resisting steel.

With reference to a specific example I will now describe how my process may with advantage be carried out in a plant as described and illustrated.

It is presumed that the raw-acid apparatus produces gallons of raw acid per minute, containing 1.00% of Cat), 1.14% of combined S0 1.14% of half-free S0 and 0.60% of completely free S0 Thus, the total S0 amounts to 2.88%. This raw-acid production is intended for three boilers, each having a capacity of 4600 cubic feet, with a cooking period of 12 hours and a period of total cycle of 15 hours. It is assumed that the raw acid has to be charged up to 3.00% of completely free S0 thatis to be strengthened by 2.40% of S0 so as to form a cooking acid containing 1.00% of CaO and 5.28% of total The raw acid liquor from the apparatus 201 flows through the pipe 207 to the absorption apparatus 203 and fills the tank 219 up to the level 225. The discs 228 are rotated rapidly and the bulk of liquor is thereby caused to circulate in the tank and the liquor is brought into intimate contact with the gas supplied to the absorption apparatus 203 through the gas conduit 208. The absorption apparatus 203 is dimensioned to hold a quantity of accumulated raw acid which bears a fixed relation to that amount of S which is to be absorbed from the gas emanating from the digester during a fixed period, and in this example, the absorption apparatus 203 is assumed to hold 530 cubic feet, 80 gallons per minute of raw acid being introduced from the raw-acid apparatus 201, a substantially equal quantity is then drawn off through the pipe 210 to the settling tank 204, where possibly present sludge may settle, and further to reclaiming tank 205. In this tank the liquor meets the relief gas rich in $0 and cooled down to 90 C., but still having a considerable pressure above atmospheric, so that it can be passed down to the bottom of the tank 205 through the gas conduit 214 and be distributed in a suitable manner into the meeting liquor. A large portion of the gas delivered into the reclaiming tank 205 will not have sufiicient time to be absorbed but will pass through the liquor and escape from tank 205 through the pipe 208. This gas passed through has a temperature of about 60 C., but its pressure head will not suffice to force the gas anew deep into a reclaiming tank. Although the degree of absorption in the reclaiming tank will be influenced by the velocity, at which the degasification of the digesters takes place, the cooling of the gas, the temperature of the liquor in the reclaiming tank, the devices used for distributing the gas into the liquor etc., usually up to 30% of all the S0 relieved from the digesters may pass through the reclaiming tank and bepassed further through the gas conduit 208. In conventional plants the gas from the reclaiming tankis often allowed to pass directly to the raw-acid plant to form raw acid liquor. The gas being relatively hot, detrimental heating of the acid in the rawacid apparatus may take place. In other plants the gas is picked up more or less completely in raw acid by being introduced into absorption towers charged with Raschig rings or similar filling bodies, over which raw acid is percolated. The absorption is not complete there either and the residual gases may then be introduced into the rawacid plant. According to the present invention, however, the gas from the reclaiming tank is passed to the highly effective absorption apparatus 203.

The liquor is strengthened in apparatus 203 only to such a value that the further strengthening in reclaiming tank 205 brings it to the desired strength of finished acid cooking liquor. According to the example chosen, the three digesters are so started at intervals that all the cooking stages are distributed over the time as evenly as possible. Three hours after starting each of the digesters a first degasification period of minutes takes place in the course of which 295 kilograms of SO =29.5 kilograms of S0 per minute are relieved.v After a further period of three hours, an additional degasification of minutes takes place, in the course of which 444 kilograms of SO kilograms of S0 per minute are relieved, and after a further period of six hours a degasification during 90 minutes takes place, when 1400 kilograms of S0 are relieved, equal to 16.4 kilograms of S0 per minute. Thus, in all 2219 kilograms of S0 are relieved from a digester. Provided that 30% of the relief gas will pass through the reclaiming tank 205 666 kilograms of S0 will pass to the absorption apparatus 203. As the SO consumption of the raw-acid plant 201 is only 8.64 kilograms per minute it will be readily understood that the irregular delivery of SO -gas from the digesters will not be suitable for use directly in the raw acid plant. And particularly so since the degasification from two boilers sometimes coincides more or less. It is thus obvious that it is an outstanding problem to master this discontinuous degasification of great amounts of S0 during short intervals of time and that high requirements are to be put on the absorption equipment and the technical arrangements for producing, nevertheless, a cooking liquor having a uniform and high content of completely free S0 Solely the gases passing through the reclaiming tank represent, during the minutes whilst a degasification is taking place, in certain cases an amount of S0 twice that to be supplied to the raw-acid plants during the same time. If these gases are allowed to pass directly to the raw-acid plant the raw acid liquor production cannot be easily controlled. Furthermore, this amount of S0 is withheld from the enrichment of the liquor to form the cooking acid liquor with the desired amount of completely free $0 In the example chosen the degasified 2219 kilograms of S0 must be absorbed in the form of completely free $0 in the raw acid produced in order to attain the required quantity of cooking acid liquor with desired 5.28% of total S0 If, instead, 30% of the degasified amount of S0 is allowed to pass from the reclaiming tank 205 to the raw acid plant to form raw acid, the percentage of total S0 of the finished cooking liquor sinks to 4.56, and the completely free S0 in the cooking liquor will only be 2.28% instead of 3%. Furthermore, in this case the amount of S0 in the supplied furnace gases must be correspondingly lowered, otherwise the content of CaO in the raw acid liquor and, as a consequence, in the cooking acid will be correspondingly increased, which means that the acid will be unduly decomposed in the digestion process, resulting in a corresponding loss of sulphur, prolonged cooking period and detrimental influence on the quality of the pulp. There is also another reason for absorbing the relief gases separately from the raw acid liquor preparation. As the gases escaping from the digesters may be considered to consist of practically pure S0 as the accompanying steam will be condensed these gases are very well suited for absorption at relatively high concentrations, whereas the ordinary furnace gases are better suited for raw acid manufacture.

What is claimed as new and desired to be secured by Letters Patent is: 1

1. A process for recovering from sulfite process digester relief gases periodically relieved from a digester and producing cooking acid liquor from raw acid liquor delivered from a raw acid liquor plant without feeding said relief gases to said plant comprising passing said gases at each time of relief directly through a body of strengthened raw sulfite acid liquor in a first absorption zone to absorb a portion of the S0 therein to form cooking acid liquor; passing the unabsorbed gases from said first absportion zone to a second absorption zone containing a body of raw sulfite acid liquor to recover the remaining S0 contained therein and to produce strengthened raw sulfite liquor, exhausting the unabsorbed gases from said second absorption zone to the atmosphere, feeding raw acid liquor, the acid liquor being prepared in said plant and independently of said zones to said second absorption zone, feeding said strengthened liquor from said second absorption zone to said first absorption zone, and accumulating a bulk of raw acid liquor in said second zone between said relief times, the absorption in said second zone being eifected by passing said gas into a closed gas space in contact with said accumulated bulk of raw acid liquor, and projecting liquor from the upper portion of said bulk into said gas space whereby intimate contact between said gas and said liquor is effected, the amount of said raw acid liquor delivered to said second absorption zone from said raw acid plant during the digester relief period being sufiicient to absorb only a portion of the S0 contained in the gases contacted therein and the amount of liquor accumulated in said bulk between said relief times being suflicient to absorb substantially all the remaining S0 contained in said gases, said digester relief gases contacting acid liquor solely in said first and second absorption zones.

2. A process as claimed in claim 1 wherein said liquor in said second absorption zone is propelled into said gas space by rapidly rotating a solid body alternately through the upper portion of the liquor and through said gas space.

3. A process as claimed in claim 1 in which the amount of raw acid liquor accumulated in said bulk together with the amount of fresh raw acid liquor supplied during said gas-emanating period of the digester being suflicient for limiting the percentage of free $0 of the strengthened acid liquor withdrawn to a value below that desired of the finished acid cooking liquor.

References Cited in the file of this patent UNITED STATES PATENTS Lindenberger June 2, 1903 Richter Jan. 23, 1917 Richter June 12, 1923 Richter June 12, 1923 Babcock Mar. 16, 1926 Richter Feb. 8, 1927 Bradley Feb. 11, 1930 Statham May 18, 1937 Kahli Nov. 12, 1940 Haglund May 5, 1953 

1. A PROCESS FOR RECOVERING SO2 FROM SULFITE PROCESS DIGESTER RELIEF GASES PERIODICALLY RELIVED FROM A DIGESTER AND PRODUCING COOKING ACID LIQUOR FROM RAW ACID LIQUOR DELIVERED FROM A RAW ACID LIQUOR PLANT WITHOUT FEEDING SAID RELIEF GASES TO SAID PLANT COMPRISING PASSING SAID GASES AT EACH TIME OF RELIEF DIRECTLY THROUGH A BODY OF STRENGTHENED RAW SULFITE ACID LIQUOR IN A FIRST ABSORPITON ZONE TO ABSORB A PORTION OF THE SO2 THERIN TO FORM COOKING ACID LIQUOR; PASSING THE UNABSORBED GASES FROM SAID FIRST ABSORPTION ZONE TO A SECOND AB SORPTION ZONE CONTAINING A BODY OF RAW SULFITE ACID LIQUOR TO RECOVER THE REMAINING SO2 CONTAINED THEEIN AND TO PRODUCE STRENGTHENED RAW SULFITE LIQUOR, EXHAUSTING THE UNABSORBED GASES FROM SAID SECOND ABSORPTION ZONE TO THE ATMOSPHERE, FEEDING RAW ACID LIQUOR, THE ACID LIQUOR BEING PREPARED IN SAID PLANT AND INDEPENDENTLY OF SAID ZONES TO SAID SECOND ABSORPTION ZONE, FEEDING SAID STRENGTHENED LIQUOR FROM SAID SECOND ABSORPTION ZONE TO SAID FIRST ABSORPTION ZONE, AND ACCUMULATING A BULK OF RAW ACID LIQUOR IN SAID SECOND ZONE BEING EFFECTED TIMES, THE ABSORPTION IN SAID SECOND ZONE BEING EFFECTED BY PASSING SAID GAS INTO A CLOSED GAS SPACE IN CONTACT WITH SAID ACCUMULATED BULK OF RAW ACID LIQUOR, AND PROJECTING LIQUOR FROM THE UPPER PORTION OF SAID BULK INTO SAID GAS SPACE WHEREBY INTIMATE CONTACT BETWEEN SAID GAS AND SAID LIQUOR IS EFFECTED, TEH AMOUNT OF SAID RAW ACID LIQUOR DELIVERED TO SAID SECOND ABSORPTION ZONE FROM SAID RAW ACID PLANT DURING THE DIGESTER RELIEF PERIOD BEING SUFFICIENT TO ABOSRB ONLY A PORTION OF THE SO2 CONTAINED IN THE GASES CONTACTED THEREIN AND THE AMOUNT OF LIQUOR ACCUMULATED IN SAID BULK BETWEEN SAID RELIEF TIMES BEING SUFFICIENT TO ABSORB SUBSTANTIALLY ALL THE REMAINING SO2 CONTAINED IN SAID GASES, SAID DIGESTER RELIEF GASES CONTACTING ACID LIQUOR SOLELY IN SAID FIRST AND SEOCND ABSORPTION ZONES. 